6V and 12V starters have a different pitch on the gear teeth, and the flywheel ring gear has a different number of teeth, IIRC. The ring gear and starter pinion gear has to match, or both get damaged. It might be possible to fit a different ring gear to the flywheel, but it's considerably easier to just swap to the correct flywheel.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!